Even when the output of EXPLAIN doesn’t show “using temporary”, a
temporary file may still be used in certain cases.

That’s not to say the query needs the temporary file to actually
resolve the query (like what you’d see from the need for a
derived table). But rather, the temporary file I’m speaking of is
due to binary logging.

In particular, you can see this easily if using InnoDB, (most
commonly) row-based binary logging, and you issue a
large transaction, say a large …

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